He defines the different wings fighting it out over Labour’s crown, he defines the disparate policy platforms promoted by those wings, he even defines the demographic challenges for Labour at the next general election.

And he will define the events of this week.

Tony Blair apologises for Iraq War mistakes and accepts invasion had part to play in rise of Islamic State

And we will defend Blair, not because he never made mistakes (though any mistakes he made were in good faith and for the best of reasons) but, more importantly, because his whole approach to politics and to government provide his party, even today, with the best template for victory and electoral relevance on offer.

Yet if Labour is comfortable declaring that it has learned from Blair’s mistakes, it is less so in admitting that it has more to learn from his triumphs.

His genius back in 1994 and 1995 now seems just plain common sense. What kind of dysfunctional entity were we as a party before then that we couldn’t acknowledge and harness the power of working class aspiration? Why did we imagine at the time that stating that it was natural and good for a family to want a nicer home, more foreign holidays, a newer car, was risky political adventurism?

Tony Blair denies Iraq allegations

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Because in the mid-1990s, that’s exactly what it was. When John Major launched his 1992 general election campaign at a DIY store, Labour sniggered disdainfully. Blair didn’t. Blair watched. And he understood what Major was doing. And he remembered.

This relatively wealthy, privately-educated barrister connected with working class voters and communities far more readily and effectively than any of his predecessors or contemporaries. He wielded that invaluable political skill of putting himself in the shoes of those less wealthy, less privileged, and asking: what would I want?

And he knew the answers, almost instinctively: a neighbourhood free from thuggish behaviour. Health care free at the point of use, beyond which point the identity of the provider was of no interest. Schools that provided the best possible standard of education, irrespective of how accountable head teachers were to the local authority. Lower taxes, better pay, more opportunity.

Blair's struggle with 'maddening' Brown

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That all this was on offer from the Labour Party says all that needs to be said about the reasons for Blair’s unrivalled electoral success.

There was certainly opposition to this approach, from those who winced at the very idea we should provide to voters what they wanted rather than what we knew was good for them. To some on the Left, a poor service provided democratically was always better than – or at least morally preferable to – a quality service provided by the private sector.

Those people are now well and truly in charge of the Labour Party (or were at the time of writing). Undoubtedly they feel genuinely angry by events leading up to, and since, the invasion of Iraq. But most of them – certainly the coterie around Jeremy Corbyn – were already antipathetic to Blair’s project long before the “War on Terror” was declared and Tony and George became best buds.

Iraq simply cemented their feelings, allowing them to condense all the resentment and anger they had felt at Blair’s damnable popularity and electoral success into one single four-letter word.

That has led to the current leadership, and most of the party membership, drawing all the wrong conclusions because they started out by inputting all the wrong data: Iraq was wrong, Iraq was a policy of Tony Blair’s, X was a policy of Tony Blair’s, therefore X was wrong.

The key moments of Blair at Iraq inquiry

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The invasion and occupation of Iraq happened because mistakes were made – mistakes by politicians, security services and military personnel. It’s right that we learn lessons from that, and we should all hope that the Chilcot Report will add more light than heat to a debate that’s already seen more than its fair share of the latter.

And yet much good also came from those mistakes, including the removal of a genocidal dictator and the establishment of a democratic regime.

Blair’s detractors will always be unable to accept such shades of grey, whether in foreign or domestic policy. They will never see past their list of grievances against Labour’s greatest election winner, a list that includes, but is certainly not limited to, Iraq.

Their refusal to acknowledge Blair’s political genius and achievements is why they will never lead Labour back to government.